Cloud identification processing is a series of tests aiming at identifying different types of cloud (or snow) present on output images acquired by SLSTR. These tests are mainly inherited from those applied on AATSR data, with new tests added mainly on the new visible channels.

The identification of cloud-affected pixels is accomplished by applying a series of tests in turn to the brightness temperature data in the 12, 11 and 3.7 micron channels, and to the reflectance data in the 1.6 micron channel, and in the visible channels. Present tests are derived from heritage of AATSR, hence new channels at 2.25 and 1.375 microns may permit the implementation of new tests. The pixel is flagged as cloudy if any one of the tests indicates the presence of cloud. The table below summarises the cloud clearing tests to be applied, including one test flagging snow-covered surfaces.

Tests name

Views

Day/night time

Land/sea

Gross cloud test

nadir and along-track views separately

both

both

Thin cirrus test

nadir and along-track views separately

both

both

Medium/high level cloud test

nadir and along-track views separately

night only

both

Fog/low stratus test

nadir and along-track views separately

night only

both

11 micron spatial coherence test

nadir and along-track views separately

both

both

1.6 micron histogram test

nadir and along-track views separately

day only

sea only

11/12 micron nadir/along-track test

both views

both

sea only

11/3.7 micron nadir/along-track test

both views

night only

sea only

Visible channel cloud test

nadir and along-track views separately

day only

land or both

Infra-red histogram test

nadir and along-track views separately

both

sea only

2.25 micron histogram test

nadir and along track views separately

day only

sea only

1.375 micron threshold test

nadir and along-track views separately

day only

both

Snow-covered surface test

nadir and along-track views separately

day only

both

Cloud identification test list

Some of the tests depend on results from the tests performed previously and hence the order in which they are applied is important.

The infra-red histogram test is applied after the other tests, and only uses those pixels that have not been flagged as cloudy by any of the preceding tests.

The 1.6 micron test operates only on pixels not previously flagged as cloudy by the gross cloud test or the thin cirrus and 11 micron spatial coherence tests, and must therefore follow these tests.

Each test makes use of a look-up table of parameters with which the brightness temperature or reflectance data is compared. Where tests are applied to along track and nadir view images separately, the parameters may be defined separately for the two cases. More generally, the comparison parameters may depend on the air mass in the line of sight, and this is implemented by allowing the tabular parameters to depend on the across track position.

Some tests are only applied under certain conditions (during day or night time only, over land or sea pixels only), as described in the table above.

Note that, in the frame of coming evolutions, two tasks of improvement of the cloud masking are planned:

The thresholds used in the current cloud masking will be tested and tuned